Lc medium

ABSTRACT

The present invention relates to liquid-crystalline (LC) media and to LC displays (LCDs) containing these media, in particular to LCDs of the twisted nematic (TN) mode, preferably to displays of the LCOS (LC on silicon) mode.

The present invention relates to liquid-crystalline (LC) media and to LCdisplays (LCDs) containing these media, in particular to LCDs of thetwisted nematic (TN) mode, preferably to displays of the LCOS (LC onsilicon) mode.

LCDs are used in many areas for the display of information. LCDs areused both for direct-view displays and for projection-type displays. Theelectrooptical modes used are, for example, the twisted nematic (TN),super twisted nematic (STN), optically compensated bend (OCB) andelectrically controlled birefringence (ECB) modes together with theirvarious modifications, as well as others. All these modes utilise anelectric field which is substantially perpendicular to the substrates orthe LC layer. Besides these modes, there are also electro-optical modesthat utilise an electric field which is substantially parallel to thesubstrates or the LC layer, such as, for example, the in-plane switching(IPS) mode (as disclosed, for example, in DE 40 00 451 and EP 0 588 568)and the fringe field switching (FFS) mode, in which a strong “fringefield” is present, i.e. a strong electric field close to the edge of theelectrodes and, throughout the cell, an electric field which has both astrong vertical component and a strong horizontal component. Theselatter two electro-optical modes in particular are used for LCDs inmodern desktop monitors and displays for TV sets and multimediaapplications. The liquid crystals according to the present invention arepreferably used in displays of this type. In general, dielectricallypositive LC media having rather lower values of the dielectricanisotropy are used in FFS displays, but in some cases LC media having adielectric anisotropy of only about 3 or even less are also used in IPSdisplays.

Especially in case of LC media for use in the expanding market of publicinformation displays (PIDs) and automotive displays a high reliabilityand a wide operating range are highly important factors. For theseapplications LC media with high clearing temperature (Tni), good LTS(Low Temperature Stability) and high reliability are therefore desired.

LCOS is a reflective micro-display mode using a liquid crystal layer ontop of a backplane made from silicon, and wherein light is reflected offthis backplane while the liquid crystal layer is switched on and off. Itis a combination of digital light processing (DLP) and LCD. The LCOSmode is typically a TN mode, and is often used for projection deviceslike e.g. projectors and projection TVs, and is also becoming widelyused for different technologies like wavelength selective switching,near-eye displays and wearable devices. The LCOS mode has the advantagesof light of weight, high resolution and mini-size.

However, in the LCOS mode, the reflective TN mode often shows someoptical defects such as the appearance of disclination lines.

The invention has the object of providing LC media, in particular for TNand LCOS mode displays, especially for active matrix displays like thoseof the TFT (thin film transistor) type, which do not exhibit thedisadvantages indicated above or only do so to a lesser extent andpreferably exhibit one or more of a wide operating range, a highclearing temperature, a high reliability, a low threshold voltage, ahigh dielectric anisotropy, a good low temperature stability (LTS), alow rotational viscosity and fast response times.

This object was achieved by providing an LC medium as described andclaimed hereinafter.

Thus, the inventors have found that it is possible to improve the imagequality of an LCOS, TN mode display, by adding a chiral dopant to the LCmedium to form a spontaneous twist in order to manage the lightdirection. This helps to increase the homogeneity of domain directionsand to suppress the appearance of disclination lines. In addition, byvarying the concentration of the chiral dopant and thus varying thechiral pitch the Voltage-Transmission (VT) curve can also be adjusted.

The displays according to the present invention are preferably addressedby an active matrix (active matrix LCDs, AMDs for short), preferably bya matrix of thin film transistors (TFTs). However, the liquid crystalsaccording to the invention can also advantageously be used in displayshaving other known addressing means.

The invention relates to an LC medium with positive dielectricanisotropy, characterised in that it contains one or more compounds offormula I, one or more compounds selected from formulae II and III, andone or more chiral dopants

wherein the individual radicals, independently of each other and on eachoccurrence identically or differently, have the following meanings

-   “alkyl” C₁₋₆-alkyl,-   R^(a1) H, CH₃ or C₂H₅,-   i, k 0, 1, 2 or 3,

-   R⁰ an unsubstituted or halogenated alkyl or alkoxy radical having 1    to 15 C atoms, where, in addition, one or more CH₂ groups in these    radicals may each be replaced, independently of one another, by    —C≡C—, —CF₂O—, —CH═CH—,

—CO—O— or —O—CO— in such a way that 0 atoms are not linked directly toone another, or denotes

-   X⁰ F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, a    halogenated alkenyl radical, a halogenated alkoxy radical or a    halogenated alkenyloxy radical having up to 6 C atoms, and-   Y⁰ H or CH₃,-   Y¹⁻⁶ H or F.

The invention further relates to the use of an LC medium as describedabove and below for electro-optical purposes, in particular for the usein LC displays, preferably in LCOS, TN or TN-TFT displays.

The invention further relates to an electro-optical LC displaycontaining an LC medium as described above and below, in particular anLCOS, TN or TN-TFT display.

The invention furthermore relates to a process for preparing an LCmedium as described above and below, comprising the steps of mixing oneor more compounds of formula I with one or more further LC compounds andoptionally one or more additives.

In the present application, all atoms also include their isotopes. Inparticular, one or more hydrogen atoms (H) may be replaced by deuterium(D), which is particularly preferred in some embodiments; a high degreeof deuteration enables or simplifies analytical determination ofcompounds, in particular in the case of low concentrations.

If R⁰ denotes an alkyl radical and/or an alkoxy radical, this may bestraight-chain or branched. It is preferably straight-chain, has 2, 3,4, 5, 6 or 7 C atoms and accordingly preferably denotes ethyl, propyl,butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexyloxyor heptyloxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl,tridecyl, tetradecyl, pentadecyl, methoxy, octyloxy, nonyloxy, decyloxy,undecyloxy, dodecyloxy, tridecyloxy or tetradecyloxy. R⁰ preferablydenotes straight-chain alkyl having 2-6 C atoms.

Oxaalkyl preferably denotes straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R⁰ denotes an alkyl radical in which one CH₂ group has been replacedby —CH═CH—, this may be straight-chain or branched. It is preferablystraight-chain and has 2 to 10 C atoms. Accordingly, it denotes, inparticular, vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-,-2-,-3- or -4-enyl, hex-1-, 2-, -3-, -4- or -5-enyl, hept-1-, -2-, -3-,-4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, 5-, -6- or -7-enyl, non-1-,-2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, dec-1-, -2-, -3-, -4-, -5-,-6-, -7-, -8- or -9-enyl.

If R⁰ denotes an alkyl or alkenyl radical which is at leastmonosubstituted by halogen, this radical is preferably straight-chain,and halogen is preferably F or Cl. In the case of polysubstitution,halogen is preferably F. The resultant radicals also includeperfluorinated radicals. In the case of monosubstitution, the fluorineor chlorine substituent may be in any desired position, but ispreferably in the ω-position.

In the formulae above and below, X⁰ is preferably F, Cl or a mono- orpolyfluorinated alkyl or alkoxy radical having 1, 2 or 3 C atoms or amono- or polyfluorinated alkenyl radical having 2 or 3 C atoms. X⁰ isparticularly preferably F, Cl, CF₃, CHF₂, OCF₃, OCHF₂, OCFHCF₃,OCFHCHF₂, OCFHCHF₂, OCF₂CH₃, OCF₂CHF₂, OCF₂CHF₂, OCF₂CF₂CHF₂,OCF₂CF₂CHF₂, OCFHCF₂CF₃, OCFHCF₂CHF₂, OCF₂CF₂CF₃, OCF₂CF₂CCIF₂,OCCIFCF₂CF₃, OCH═CF₂ or CH═CF₂, very particularly preferably F or OCF₃,furthermore CF₃, OCF═CF₂, OCHF₂ or OCH═CF₂.

Particular preference is given to compounds in which X⁰ denotes F orOCF₃, preferably F.

FIG. 1a and FIG. 1b schematically and exemplarily illustrate an LCOSdisplay according to the present invention, comprising asilicon-containing, e.g. CMOS, backplane (1), a reflective coating (2),preferably in the shape of a pixel array, first (3) and second (4)alignment layers, providing e.g. planar alignment, a layer (5) of an LCmedium with positive dielectric anisotropy as defined above and below, atransparent, e.g. ITO, electrode layer (6), and a transparent substrate(7) e.g. of glass or transparent plastic.

In the non-addressed or off-state (a), i.e. with no voltage applied, theLC molecules (8) of the LC layer (5) exhibit a planar, twisted nematicalignment (induced by the chiral dopant). In the addressed or on-state(b), i.e. upon application of a voltage, the LC molecules (8) reorientinto vertical alignment.

In the LC media and displays according to the present invention, achiral dopant is added to the LC medium to form a spontaneous twist inorder to manage the light direction. The control of the twist angle andthus the pitch length is important to prevent under/over twist and toreach the best optical performance.

The threshold voltage (Vth) of the 900 TN-LCD is given by the equation(1) wherein pitch>λ (wavelength of light propagating inside a LCmaterial):

$\begin{matrix}{V_{th} = {\pi\sqrt{\frac{K_{11} + {0.25\left( {K_{33} - {2K_{21}}} \right)} + {2K_{22}/\left( {p/d} \right)}}{\varepsilon_{0}\Delta\varepsilon}}}} & (1)\end{matrix}$

Here the (p/d) ratio represent the chiral pitch-to-cell gap, Δε and γ1represent the dielectric anisotropy and rotational viscosity; K11, K22,K33 represent elastic constants of the splay, twist, and benddeformations respectively.

From equation (1) above it follows that a shorter pitch will lead to aright shift of the VT curve, whereas a longer pitch will lead to a leftshift of the VT curve with better contrast ratio. However, a longerpitch will also increase the risk of disclination lines appearing,caused by weak domain direction.

The chiral dopants are preferably selected from the group consisting ofcompounds from Table C below, very preferably from the group consistingof R- or S-1011, R- or S-2011, R- or S-3011, R- or S-4011, and R- orS-5011:

Very preferred are chiral dopants of formula S-4011.

Preferably the LC medium contains one or two, more preferably one,chiral dopants, in a concentration from 0.01 to 1% by weight, preferablyin a concentration from 0.1 to 0.72%, more preferably from 0.2 to 0.72%by weight and very preferably from 0.35 to 0.72% by weight.

The helical twisting power and amount of the chiral dopant in the LCmedium are preferably selected such that the ratio d/p in the displayaccording to the present invention preferably from 0.015 to 0.2, morepreferably from 0.03 to 0.2, and very preferably from 0.05 to 0.2.

Preferably the twist angle of the helical twist induced in the LC mediumby the chiral dopants (before applying a voltage) is from 60 to 120°,more preferably from 80 to 100°, very preferably 90°.

Preferably the pitch of the helical twist induced in the LC medium bythe chiral dopants is from 15 to 100 μm, more preferably from 15 to 50μm, and very preferably from 15 to 30 μm.

Preferably the ratio d/p in a display according to the present inventionis from 0.015 to 0.2, more preferably from 0.03 to 0.2, most preferablyfrom 0.05 to 0.2.

In the compounds of formula I R¹ and R² are preferably selected fromethyl, propyl, butyl and pentyl, all of which are straight-chain.

Preferably the LC medium comprises one or more compounds of formula Iselected from the group consisting of the following subformulae

wherein “alkyl” has the meaning given in formula I and preferablydenotes C₂H₅, n-C₃H₇, n-C₄H₉ or n-C₅H₁₁, in particular n-C₃H₇.

Very preferably the LC medium comprises one or more compounds of formulaI selected from the group consisting of the following subformulae:

Very preferred are compounds of formula Ia1, Ib1 and Ic1.

The concentration of the compounds of formula I and its subformulae inthe LC medium is preferably from 15 to 70%, very preferably from 20 to60%.

Preferably the LC medium contains 1, 2 or 3 compounds of formula I orits subformulae.

In the compounds of formulae II and III and their subformulae R⁰preferably denotes straight-chain alkyl having 1 to 6 C atoms, inparticular methyl, ethyl or propyl, furthermore alkenyl having 2 to 6 Catoms, in particular vinyl, 1E-propenyl, 1E-butenyl, 3-butenyl,1E-pentenyl, 3E-pentenyl or 4-pentenyl.

Preferably the LC medium comprises one or more compounds of formula IIwherein Y⁰ is H, preferably selected from the group consisting of thefollowing subformulae

in which R⁰ and X⁰ have the meanings given above.

Preferred compounds are those of formula I11, I12 and I13, verypreferred those of formula II1 and II2.

In the compounds of formulae II1 to II7R⁰ preferably denotes alkylhaving 1 to 6 C atoms, very preferably ethyl or propyl, and X⁰preferably denotes F or OCF₃, very preferably F.

In another preferred embodiment the LC medium comprises one or morecompounds of formula II wherein Y⁰ is CH₃, preferably selected from thegroup consisting of the following subformulae

in which R⁰ and X⁰ have the meanings given above.

Preferred compounds are those of formula IIA1, IIA2 and IIA3, verypreferred those of formula IIA1 and IIA2.

In the compounds of formulae IIA1 to IIA7R⁰ preferably denotes alkylhaving 1 to 6 C atoms, very preferably ethyl or propyl, and X⁰preferably denotes F or OCF₃, very preferably F.

Further preferably the LC medium comprises one or more compounds offormula III wherein Y⁰ is H, preferably selected from the groupconsisting of the following subformulae

in which R⁰ and X⁰ have the meanings given above.

Preferred compounds are those of formula III1, III4, III6, III16, III19and III20.

In the compounds of formulae III1 to III21R⁰ preferably denotes alkylhaving 1 to 6 C atoms, very preferably ethyl or propyl, X⁰ preferablydenotes F or OCF₃, very preferably F, and Y² preferably denotes F.

In another preferred embodiment the LC medium comprises one or morecompounds of formula III wherein Y⁰ is CH₃, preferably selected from thegroup consisting of the following subformulae

in which R⁰ and X⁰ have the meanings given above

Preferred compounds are those of formula IIIA1, IIIA4, IIIA6, IIIA16,IIIA19 and IIIA20.

In the compounds of formulae IIIA1 to IIIA21 R⁰ preferably denotes alkylhaving 1 to 6 C atoms, very preferably ethyl or propyl, X⁰ preferablydenotes F or OCF₃, very preferably F, and Y² preferably denotes F.

The concentration of an individual compound of formula II and itssubformulae in the LC medium is preferably from 1 to 15% by weight. Thetotal concentration of the compounds of formula II in the LC medium ispreferably from 5 to 25% by weight.

The concentration of an individual compound of formula III in the LCmedium s is preferably from 1 to 15% by weight. The total concentrationof the compounds of formula III is preferably from 2 to 25% by weight.

Preferably the LC medium contains 1 to 8 compounds of formula II and/orformula III or their subformulae.

Preferably the LC medium contains at least one compound of formula II orits subformulae and at least one compound of formula II or itssubformulae.

Further preferred embodiments are indicated below, including anycombination thereof:

-   -   The medium additionally comprises one or more compounds selected        from the following formulae:

-   -   in which    -   R⁰, X⁰ and Y¹⁻⁴ have the meanings indicated above, and    -   Z⁰ denotes —C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—,        —CF₂CH₂—, —CH₂O—, —OCH₂—, —COO— or —OCF₂—, in formulae V and VI        also a single bond, in formulae V and VIII also —CF₂O—,    -   r denotes 0 or 1, and    -   s denotes 0 or 1;    -   The medium comprises one or more compounds of the formula IV        selected from the following subformulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.    -   R⁰ preferably denotes alkyl having 1 to 6 C atoms. X⁰ preferably        denotes F or OCF₃, furthermore OCF═CF₂ or Cl;    -   The medium comprises one or more compounds of the formula IVa        selected from the following subformula:

-   -   in which R⁰ has the meanings indicated above and is preferably        propyl or pentyl.    -   The medium comprises one or more compounds of the formula IVc        selected from the following subformula:

-   -   in which R⁰ has the meanings indicated above and is preferably        propyl or pentyl.    -   The medium comprises one or more compounds of the formula V        selected from the following subformulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.    -   R⁰ preferably denotes alkyl having 1 to 6 C atoms. X⁰ preferably        denotes F and OCF₃, furthermore OCHF₂, CF₃, OCF═CF₂ and OCH═CF₂;    -   The medium comprises one or more compounds of the formula VI        selected from the following subformulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.    -   R⁰ preferably denotes alkyl having 1 to 6 C atoms. X⁰ preferably        denotes F, furthermore OCF₃, CF₃, CF═CF₂, OCHF₂ and OCH═CF₂;    -   The medium comprises one or more compounds of the formula VII        selected from the following subformulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.    -   R⁰ preferably denotes alkyl having 1 to 6 C atoms. X⁰ preferably        denotes F, furthermore OCF₃, OCHF₂ and OCH═CF₂.    -   The medium additionally comprises one or more compounds selected        from the following formulae:

-   -   in which X⁰ has the meanings indicated above, and    -   L denotes H or F,    -   “alkyl” denotes C₁₋₆-alkyl,    -   R′ denotes C₁₋₆-alkyl or C₁₋₆-alkoxy    -   R″ denotes C₁₋₆-alkyl, C₁₋₆-alkoxy or C₂₋₆-alkenyl, and    -   “alkenyl” and “alkenyl*” each, independently of one another,        denote C₂₋₆-alkenyl.    -   The medium comprises one or more compounds of formulae IX-XII        selected from the following subformulae:

-   -   wherein “alkyl” has the meanings given above.

Particular preference is given to the compounds of the formulae IXa,IXb, IXc, Xa, Xb, XIa and XIIa. In the formula IXa the groups “alkyl”preferably, independently of one another, denote C₂H₅, n-C₃H₇, n-C₄H₉ orn-C₅H₁₁, very preferably C₂H₅ or n-C₃H₇. In the formulae XIa and XIb“alkyl” preferably denotes CH₃, C₂H₅ or n-C₃H₇, very preferably CH₃.

-   -   The medium comprises one or more compounds of formulae IX-XII        selected from the following subformulae;

-   -   The medium comprises one or more compounds of the following        formula:

-   -   in which L¹ and L² denote independently of one another H or F,        in which R¹ and R² independently of each other denote alkyl or        alkoxy having 1 to 6 C atoms or alkenyl having 2 to 6 C atoms.

Preferred compounds of formula XIII are those wherein L¹ and L² are H.

Further preferred compounds of formula XIII are those wherein L¹ and L²are F.

Further preferred compounds of formula XIII are those wherein R¹ isalkyl with 1 to 6 C atoms, preferably methyl, ethyl or propyl, and R² isalkoxy with 1 to 6 C atoms, preferably methoxy, ethoxy or propoxy.

-   -   The medium comprises one or more compounds of the following        formula:

in which R¹ and R² independently of each other denote alkyl having 1 to6 C atoms.

Preferred compounds of the formula XIV are selected from the followingsubformulae.

-   -   Very preferred are compounds of formula XIV2.    -   The medium additionally comprises one or more compounds selected        from the following formula:

-   -   in which R³ and R⁴ each, independently of one another, denote        n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having        up to 6 C atoms, and preferably each, independently of one        another, denote alkyl having 1 to 6 C atoms or alkenyl having 2        to 6 C atoms.    -   The medium comprises one or more compounds of the formula XV        selected from the following subformulae:

-   -   in which “alkyl” has the meaning indicated above, and preferably        denotes methyl, ethyl or propyl.    -   The medium comprises one or more compounds of the formula XV        selected from the following subformulae:

-   -   Very preferred are compounds of formula XVd1 and XVe1.    -   The medium comprises one or more compounds of the formula XVI,

-   -   in which L, R³ and R⁴ have the meanings indicated above and        preferably each, independently of one another, denote alkyl        having 1 to 6 C atoms.

Particularly preferred compounds of the formula XVI are those of thesub-formulae

-   -   in which    -   alkyl and alkyl* each, independently of one another, denote a        straight-chain alkyl radical having 1-6 C atoms, in particular        ethyl, propyl or pentyl,    -   alkenyl and alkenyl* each, independently of one another, denote        a straight-chain alkenyl radical having 2-6 C atoms, in        particular CH₂═CHC₂H₄, CH₃CH═CHC₂H₄, CH₂═CH and CH₃CH═CH.    -   Particular preference is given to the compounds of the formulae        XVIb, XVIc and XVIg. Very particular preference is given to the        compounds of the formulae

-   -   Very preferred are compounds of formula XVIc2, XVIg1 and XVIg2;    -   The medium comprises one or more compounds of the following        formula:

-   -   in which R³ and R⁴ have the meanings indicated above and        preferably each, independently of one another, denote alkyl        having 1 to 6 C atoms. L denotes H or F;    -   The medium additionally comprises one or more compounds selected        from the following formulae:

-   -   in which R⁰ and X⁰ each, independently of one another, have one        of the meanings indicated above, and Y¹⁻⁴ each, independently of        one another, denote H or F. X⁰ is preferably F, Cl, CF₃, OCF₃ or        OCHF₂. R⁰ preferably denotes alkyl, alkoxy, oxaalkyl,        fluoroalkyl or alkenyl, each having up to 6 C atoms.

Very preferably the mixture according to the invention comprises one ormore compounds of the formula XXIa,

-   -   in which R⁰ has the meanings indicated above. R⁰ preferably        denotes straight-chain alkyl, in particular ethyl, n-propyl,        n-butyl and n-pentyl and very particularly preferably n-propyl.        The compound(s) of the formula XXI, in particular of the formula        XXIa, is (are) preferably employed in the mixtures according to        the invention in amounts of 1-15% by weight, particularly        preferably 2-10% by weight.

Further preferably the mixture according to the invention comprises oneor more compounds of the formula XXIIIa,

-   -   in which R⁰ has the meanings indicated above. R⁰ preferably        denotes straight-chain alkyl, in particular ethyl, n-propyl,        n-butyl and n-pentyl and very particularly preferably n-propyl.        The compound(s) of the formula XXIII, in particular of the        formula XXIIIa, is (are) preferably employed in the mixtures        according to the invention in amounts of 0.5-10% by weight,        particularly preferably 0.5-5% by weight.    -   The medium additionally comprises one or more compounds of the        formula XXIV,

-   -   in which R⁰, X⁰ and Y¹⁻⁶ have the meanings indicated in formula        I, s denotes 0 or 1, and

denotes

-   -   In the formula XXIV, X⁰ may also denote an alkyl radical having        1-6 C atoms or an alkoxy radical having 1-6 C atoms. The alkyl        or alkoxy radical is preferably straight-chain.

R⁰ preferably denotes alkyl having 1 to 6 C atoms. X⁰ preferably denotesF;

-   -   The compounds of the formula XXIV are preferably selected from        the following formulae:

-   -   in which R⁰, X⁰ and Y¹ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 6 C atoms. X⁰ preferably        denotes F, and Y¹ is preferably F;

is preferably

-   -   R⁰ is straight-chain alkyl or alkenyl having 2 to 6 C atoms;    -   The medium comprises one or more compounds of the following        formulae:

-   -   in which R³ and X⁰ have the meanings indicated above. R³        preferably denotes alkyl having 1 to 6 C atoms. X⁰ preferably        denotes F or Cl. In the formula XXIV, X⁰ very particularly        preferably denotes Cl.    -   The medium comprises one or more compounds of the following        formulae:

-   -   in which R³ and X⁰ have the meanings indicated above. R³        preferably denotes alkyl having 1 to 6 C atoms. X⁰ preferably        denotes F. The medium according to the invention particularly        preferably comprises one or more compounds of the formula XXIX        in which X⁰ preferably denotes F. The compound(s) of the        formulae XXVI-XXIX is (are) preferably employed in the mixtures        according to the invention in amounts of 1-20% by weight,        particularly preferably 1-15% by weight. Particularly preferred        mixtures comprise at least one compound of the formula XXIX.    -   Very preferably the mixture according to the invention comprises        one or more compounds of the formula XXIXa,

-   -   in which R³ has the meanings indicated above, and preferably        denotes straight-chain alkyl, in particular ethyl, n-propyl,        n-butyl and n-pentyl and very particularly preferably n-propyl.        The compound(s) of the formula XXIXa is (are) preferably        employed in the mixtures according to the invention in amounts        of 1-15% by weight, particularly preferably 2-10% by weight.    -   The medium comprises one or more compounds of the following        formula

-   -   in which R³ and R⁴ have the meanings indicated above, and        preferably denote, independently of one another, alkyl having 1        to 6 C atoms, very preferably methyl, ethyl or n-propyl.

Preferred compounds of formula XXX are those of formula XXXa

-   -   The medium comprises one or more compounds of the following        formula

-   -   in which R³ has the meanings indicated above, and R⁵ has one of        the meanings of R⁴ or X⁰ as given above.    -   R³ preferably denotes alkyl having 1 to 6 C atoms, very        preferably methyl, ethyl or n-propyl, or alkenyl having 2 to 6 C        atoms, very preferably vinyl. R⁵ preferably denotes alkyl having        1 to 6 C atoms, very preferably methyl, ethyl or n-propyl, or        trifluoromethyl or trifluoromethoxy.    -   Preferred compounds of formula XXXI are selected from the group        consisting of the following subformulae

-   -   wherein alkyl and alkenyl have the meanings given above.    -   Very preferred compounds of formula XXXI are selected from the        group consisting of the following subformulae

Further preferred LC media are selected from the following preferredembodiments, including any combination thereof:

-   -   The medium comprises one or more compounds of the formula IV,        preferably selected from formulae IVa or IVc, very preferably        from formula IVa1 or IVc1. The individual concentration of each        of these compounds is preferably from 1 to 15% by weight. The        total concentration of these compounds is preferably from 5 to        25% by weight.    -   The medium comprises one or more compounds of the formula IX,        preferably selected from formulae IXa, very preferably from        formulae IXa1 and IXa2. The individual concentration of each of        these compounds is preferably from 1 to 15% by weight. The total        concentration of these compounds is preferably from 5 to 25% by        weight.    -   The medium comprises one or more compounds of the formula XI,        preferably selected from formulae XIa and XIb. The individual        concentration of each of these compounds is preferably from 1 to        20% by weight. The total concentration of these compounds is        preferably from 5 to 25% by weight.    -   The medium comprises one or more compounds of the formula XII,        preferably of formula XIIa. The total concentration of these        compounds is preferably from 3 to 20% by weight.    -   The medium comprises one or more compounds of the formula XV,        preferably selected from formulae formulae XVd and XVe, very        preferably from formulae XVd1 and XVe1. The individual        concentration of each of these compounds is preferably from 1 to        10% by weight. The total concentration of these compounds is        preferably from 2 to 15% by weight.    -   The medium comprises one or more compounds of the formula XVIc,        preferably of formula XVIc2. The concentration of these        compounds is preferably from 1 to 15% by weight.    -   The medium comprises one or more compounds of the formula XVIg,        preferably of the formula XVIg1 and/or XVIg2. The concentration        of these compounds is preferably from 5 to 25% by weight.    -   The medium comprises one or more compounds of the formula XVIIa        or XVIIb. The concentration of these compounds is preferably        from 0.5 to 5% by weight.    -   The medium comprises one or more compounds of the formula XXI,        preferably of the formula XXIa. The concentration of these        compounds is preferably from 0.5 to 8% by weight.    -   The medium comprises one or more compounds of the formula XXIII,        preferably of the formula XXIIIa. The concentration of these        compounds is preferably from 0.5 to 5% by weight.    -   The medium comprises one or more compounds of the formula XXIX,        preferably of the formula XXIXa. The concentration of these        compounds is preferably from 2 to 10% by weight.    -   The medium comprises one or more compounds of the formula XXX,        preferably of the formula XXXa. The concentration of these        compounds is preferably from 2 to 10% by weight.    -   The medium comprises one or more compounds of the formula XXXI,        preferably of the formula XXXIa and/or XXXIb. The concentration        of these compounds is preferably from 2 to 10% by weight.    -   The medium comprises one or more compounds of formula I and one        or more compounds selected from the group consisting of the        formulae II, III, IV, IX, XI, XII, XV, XVI, XXI, XXIX and XXXI.    -   The medium comprises one or more compounds of formula I and one        or more compounds selected from the group consisting of the        formulae II1, II2, II3, III1, III4, III6, III16, III19, III20,        IVa, IVc, IXa, IXb, XIa, XIb, XIIa, XVd, XVe, XVIc, XXIa, XXIXa        and XXXIa.    -   The proportion of compounds of the formulae II, III, IV, IX, XI,        XII, XV, XVI, XXI, XXIX and XXX in the mixture as a whole is 90        to 99% by weight.    -   The proportion of compounds of the formulae II1, II2, II3, III1,        III4, III6, III16, III19, III20, IVa, IVc, IXa, IXb, XIa, XIb,        XIIa, XVd, XVe, XVIc, XXIa, XXIXa and XXXIa in the mixture as a        whole is 90 to 99% by weight.

The term “alkyl” or “alkyl*” in this application encompassesstraight-chain and branched alkyl groups having 1-6 carbon atoms, inparticular the straight-chain groups methyl, ethyl, propyl, butyl,pentyl and hexyl. Groups having 2-5 carbon atoms are generallypreferred.

The term “alkenyl” or “alkenyl*” encompasses straight-chain and branchedalkenyl groups having 2-6 carbon atoms, in particular the straight-chaingroups. Preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₆-3E-alkenyl,in particular C₂-C₆-1E-alkenyl. Examples of particularly preferredalkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl,1E-hexenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl and 5-hexenyl. Groups having up to 5 carbon atoms aregenerally preferred, in particular CH₂═CH, CH₃CH═CH.

The term “fluoroalkyl” preferably encompasses straight-chain groupshaving a terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

The term “oxaalkyl” or “alkoxy” preferably encompasses straight-chainradicals of the formula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and meach, independently of one another, denote 1 to 6. m may also denote 0.Preferably, n=1 and m=1-6 or m=0 and n=1-3.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio between the elasticconstants k₃₃ (bend) and k₁₁ (splay) compared with alkyl and alkoxyradicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and lower values of k₃₃/k₁₁ compared withalkyl and alkoxy radicals. The mixtures according to the invention aredistinguished, in particular, by high Δε values and thus havesignificantly faster response times than the mixtures from the priorart.

The optimum mixing ratio of the compounds of the above-mentionedformulae depends substantially on the desired properties, on the choiceof the components of the above-mentioned formulae and on the choice ofany further components that may be present.

Suitable mixing ratios within the range indicated above can easily bedetermined from case to case.

The total amount of compounds of the above-mentioned formulae in the LCmedia according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimisation of various properties. However, the observed effect on thedesired improvement in the properties of the medium is generallygreater, the higher the total concentration of compounds of theabove-mentioned formulae.

In a particularly preferred embodiment, the LC media according to theinvention comprise compounds of the formulae IV to VIII (preferably IVand V) in which X⁰ denotes F, OCF₃, OCHF₂, OCH═CF₂, OCF═CF₂ orOCF₂—CF₂H. A favourable synergistic action with the compounds of theformulae IA, IIA, IB and IIB results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaeIA or IIA and IB or IIB are distinguished by their low thresholdvoltage.

The individual compounds of the above-mentioned formulae and thesubformulae thereof which can be used in the LC media according to theinvention are either known or can be prepared analogously to the knowncompounds.

The invention also relates to electro-optical displays, such as, forexample, TN or MLC displays, having two plane-parallel outer plates,which, together with a frame, form a cell, integrated non-linearelements for switching individual pixels on the outer plates, and anematic LC mixture having positive dielectric anisotropy and highspecific resistance located in the cell, wherein the a nematic LCmixture is an LC medium according to the present invention as describedabove and below.

The LC media according to the invention enable a significant broadeningof the available parameter latitude. The achievable combinations ofclearing point, viscosity at low temperature, thermal and UV stabilityand high optical anisotropy are far superior to previous materials fromthe prior art.

The LC media according to the invention are suitable for mobileapplications and TFT applications, such as, for example, mobiletelephones and PDAs.

Furthermore, the LC media according to the invention are particularlysuitably for use in FFS and IPS displays.

The LC media according to the invention preferably retain the nematicphase down to −20° C. very preferably down to −30° C., most preferablydown to −40° C. The LC media according to the invention preferably havea clearing point ≥85° C., very preferably 95° C., most preferably 105°C.

The LC media according to the invention preferably have a rotationalviscosity γ₁ of 130 mPa·s, very preferably 115 mPa·s, enabling excellentMLC displays having fast response times to be achieved. The rotationalviscosities are determined at 20° C.

In a preferred embodiment, the dielectric anisotropy Δε of the LC mediaaccording to the invention at 20° C. is preferably ≥+4, very preferably≥+6, most preferably ≥+8.

The birefringence Δn of the LC media according to the invention at 20°C. is preferably in the range of from 0.080 to 0.150, more preferablyfrom 0.090 to 0.140, particularly preferably 0.100 to 0.130.

The nematic phase range of the LC media according to the inventionpreferably has a width of at least 100°, more preferably of at least110° C., in particular at least 130°. This range preferably extends atleast from −25° to +105° C.

It goes without saying that, through a suitable choice of the componentsof the LC media according to the invention, it is also possible forhigher clearing points (for example above 100° C.) to be achieved athigher threshold voltages or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain LC media having a higher Δε and thus lowthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German patent 30 22 818), lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistance values to be achieved using the mixturesaccording to the invention at the first minimum than in the case of LCmedia comprising cyano compounds. Through a suitable choice of theindividual components and their proportions by weight, the personskilled in the art is able to set the birefringence necessary for apre-specified layer thickness of the MLC display using simple routinemethods.

Measurements of the voltage holding ratio (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that LC media according to theinvention comprising compounds of the formulae ST-1, ST-2, RV, IA and IBexhibit a significantly smaller decrease in the HR on UV exposure thananalogous mixtures comprising cyanophenylcyclohexanes of the formula

or esters of the formula

instead of the compounds of the formulae I ST-1, ST-2, RV, IA and IB.

The light stability and UV stability of the LC media according to theinvention are considerably better, i.e. they exhibit a significantlysmaller decrease in the HR on exposure to light, heat or UV.

The construction of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the usual design for displays of this type. The termusual design is broadly drawn here and also encompasses all derivativesand modifications of the MLC display, in particular including matrixdisplay elements based on poly-Si TFTs or MIM.

A significant difference between the displays according to the inventionand the hitherto conventional displays based on the twisted nematic cellconsists, however, in the choice of the LC parameters of the LC layer.

The LC media which can be used in accordance with the invention areprepared in a manner conventional per se, for example by mixing one ormore compounds of Claim 1 with one or more compounds of the formulaeIV-XXXI or with further LC compounds and/or additives. In general, thedesired amount of the components used in lesser amount is dissolved inthe components making up the principal constituent, advantageously atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and to remove the solvent again, for example by distillation,after thorough mixing.

The LC media may also comprise further additives known to the personskilled in the art and described in the literature, such as, forexample, polymerisation initiators, inhibitors, surface-activesubstances, light stabilisers, antioxidants, e.g. BHT, TEMPOL,microparticles, free-radical scavengers, nanoparticles, etc. Forexample, 0-15% of pleochroic dyes or chiral dopants or initiators likeIrgacure651@ or Irgacure907® can be added. Suitable stabilisers anddopants are mentioned below in Tables C and D.

In a preferred embodiment of the present invention the LC media containone or more further stabilisers, preferably selected from the groupconsisting of the following formulae

wherein the individual radicals, independently of each other and on eachoccurrence identically or differently, have the following meanings

-   R^(a-d) straight-chain or branched alkyl with 1 to 10, preferably 1    to 6, very preferably 1 to 4 C atoms, most preferably methyl,-   X^(S) H, CH₃, OH or O.,-   A^(S) straight-chain, branched or cyclic alkylene with 1 to 20 C    atoms which is optionally substituted,-   n an integer from 1 to 6, preferably 3.

Preferred stabilisers of formula S3 are selected from formula S3A

wherein n2 is an integer from 1 to 12, and wherein one or more H atomsin the group (CH₂)_(n2) are optionally replaced by methyl, ethyl,propyl, butyl, pentyl or hexyl.

Very preferred stabilisers are selected from the group consisting of thefollowing formulae

In a preferred embodiment the LC medium comprises one or morestabilisers selected from the group consisting of formulae S1-1, S2-1,S3-1, S3-1 and S3-3.

In a preferred embodiment the LC medium comprises one or morestabilisers selected from Table D.

Preferably the proportion of stabilisers, like those of formula S1-S3,in the LC medium is from 10 to 500 ppm, very preferably from 20 to 100ppm.

In another preferred embodiment an LC medium contains one or morepolymerisable compounds, preferably selected from polymerizablemesogenic compounds. These LC media are suitable for use in displays ofthe polymer stabilized (PS) mode, like the PS-TN mode.

Another preferred embodiment of the present invention thus relates to aPS-TN display comprising an LC medium as described above and below.

Preferably the LC medium according to this preferred embodiment containsone or more polymerisable compounds of formula M

R^(a)—B¹—(Z^(b)—B²)_(m)—R^(b)  M

in which the individual radicals, on each occurrence identically ordifferently, and each, independently of one another, have the followingmeaning:

-   R^(a) and R^(b) P, P-Sp-, H, F, Cl, Br, I, —CN, —NO₂, —NCO, —NCS,    —OCN, —SCN, SF₅ or straight-chain or branched alkyl having 1 to 25 C    atoms, in which, in addition, one or more non-adjacent CH₂ groups    may each be replaced, independently of one another, by    —C(R⁰)═C(R⁰⁰)—, —C═C—, —N(R⁰⁰)—, —O—, —S—, —CO—, —COO—, —O—CO—,    —O—CO—O— in such a way that O and/or S atoms are not linked directly    to one another, and in which, in addition, one or more H atoms may    be replaced by F, C, Br, I, CN, P or P-Sp-, where, if B¹ and/or B²    contain a saturated C atom, R^(a) and/or R^(b) may also denote a    radical which is spiro-linked to this saturated C atom,    -   wherein at least one of the radicals R^(a) and R^(b) denotes or        contains a group P or P-Sp-,-   P a polymerizable group,-   Sp a spacer group or a single bond,-   B¹ and B² an aromatic, heteroaromatic, alicyclic or heterocyclic    group, preferably having 4 to 25 ring atoms, which may also contain    fused rings, and which is unsubstituted, or mono- or polysubstituted    by L,-   Z^(b)—O—, —S—, —CO—, —CO—O—, —OCO—, —O—CO—O—, —OCH₂—, —CH₂O—,    —SCH₂—, —CH₂S—, —CF₂O—, —OCF₂—, —CF₂S—, —SCF₂—, —(CH₂)_(n1)—,    —CF₂CH₂—, —CH₂CF₂—, —(CF₂)_(n1)—, —CH═CH—, —CF═CF—, —C≡C—,    —CH═CH—COO—, —OCO—CH═CH—, CR⁰R⁰⁰ or a single bond,-   R⁰ and R⁰⁰ each, independently of one another, denote H or alkyl    having 1 to 12 C atoms,-   m denotes 0, 1, 2, 3 or 4,-   n1 denotes 1, 2, 3 or 4,-   L P, P-Sp-, OH, CH₂OH, F, Cl, Br, I, —CN, —NO₂, —NCO, —NCS, —OCN,    —SCN, —C(═O)N(R^(x))₂, —C(═O)Y¹, —C(═O)R^(x), —N(R^(x))₂, optionally    substituted silyl, optionally substituted aryl having 6 to 20 C    atoms, or straight-chain or branched alkyl, alkoxy, alkylcarbonyl,    alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 25    C atoms, in which, in addition, one or more H atoms may be replaced    by F, C, P or P-Sp-,-   Y¹ denotes halogen,-   R^(x) denotes P, P-Sp-, H, halogen, straight-chain, branched or    cyclic alkyl having 1 to 25 C atoms, in which, in addition, one or    more non-adjacent CH₂ groups may be replaced by —O—, —S—, —CO—,    —CO—O—, —O—CO—, —O—CO—O— in such a way that 0 and/or S atoms are not    linked directly to one another, and in which, in addition, one or    more H atoms may be replaced by F, C, P or P-Sp-, an optionally    substituted aryl or aryloxy group having 6 to 40 C atoms, or an    optionally substituted heteroaryl or heteroaryloxy group having 2 to    40 C atoms.

Particularly preferred compounds of the formula M are those in which B¹and B² each, independently of one another, denote 1,4-phenylene,1,3-phenylene, naphthalene-1,4-diyl, naphthalene-2,6-diyl,phenanthrene-2,7-diyl, 9,10-dihydro-phenanthrene-2,7-diyl,anthracene-2,7-diyl, fluorene-2,7-diyl, coumarine, flavone, where, inaddition, one or more CH groups in these groups may be replaced by N,cyclohexane-1,4-diyl, in which, in addition, one or more non-adjacentCH₂ groups may be replaced by 0 and/or S, 1,4-cyclohexenylene,bicycle[1.1.1]pentane-1,3-diyl, bicyclo[2.2.2]octane-1,4-diyl,spiro[3.3]heptane-2,6-diyl, piperidine-1,4-diyl,decahydronaphthalene-2,6-diyl, 1,2,3,4-tetrahydronaphthalene-2,6-diyl,indane-2,5-diyl or octahydro-4,7-methanoindane-2,5-diyl, where all thesegroups may be unsubstituted or mono- or polysubstituted by L as definedabove.

Particularly preferred compounds of the formula M are those in which B¹and B² each, independently of one another, denote 1,4-phenylene,1,3-phenylene, naphthalene-1,4-diyl or naphthalene-2,6-diyl.

The polymerizable group P is a group which is suitable for apolymerization reaction, such as, for example, free-radical or ionicchain polymerization, polyaddition or polycondensation, or for apolymer-analogous reaction, for example addition or condensation onto amain polymer chain. Particular preference is given to groups for chainpolymerization, in particular those containing a C═C double bond or—C≡C— triple bond, and groups which are suitable for polymerization withring opening, such as, for example, oxetane or epoxide groups.

Preferred groups P are selected from the group consisting ofCH₂═CW¹—CO—O—,

CH₂═CW²—(O)_(k3)—, CW¹═CH—CO—(O)_(k3)—, CW¹═CH—CO—NH—, CH₂═CW¹—CO—NH—,CH₃—CH═CH—O—, (CH₂═CH)₂CH—OCO—, (CH₂═CH═CH₂)₂CH—OCO—, (CH₂═CH)₂CH—O—,(CH₂═CH—CH₂)₂N—, (CH₂═CH—CH₂)₂N—CO—, HO—CW²W³—, HS—CW²W³—, HW²N—,HO—CW²W³—NH—, CH₂═CW¹—CO—NH—, CH₂═CH—(COO)_(k1)-Phe-(O)_(k2)—,CH₂═CH—(CO)_(k1)-Phe-(O)_(k2)—, Phe-CH═CH—, HOOC—, OCN— and W⁴W⁵W⁶Si—,in which W¹ denotes H, F, Cl, CN, CF₃, phenyl or alkyl having 1 to 5 Catoms, in particular H, F, Cl or CH₃, W² and W³ each, independently ofone another, denote H or alkyl having 1 to 5 C atoms, in particular H,methyl, ethyl or n-propyl, W⁴, W⁵ and W⁶ each, independently of oneanother, denote Cl, oxaalkyl or oxacarbonylalkyl having 1 to 5 C atoms,W⁷ and W⁸ each, independently of one another, denote H, Cl or alkylhaving 1 to 5 C atoms, Phe denotes 1,4-phenylene, which is optionallysubstituted by one or more radicals L as defined above which are otherthan P-Sp-, k₁, k₂ and k₃ each, independently of one another, denote 0or 1, k₃ preferably denotes 1, and k₄ denotes an integer from 1 to 10.

Very preferred groups P are selected from the group consisting ofCH₂═CW¹—CO—O—,

CH₂═CW²—O—, CH₂═CW²—, CW¹═CH—CO—(O)_(k3)—, CW¹═CH—CO—NH—,CH₂═CW¹—CO—NH—, (CH₂═CH)₂CH—OCO—, (CH₂═CH—CH₂)₂CH—OCO—, (CH₂═CH)₂CH—O—,(CH₂═CH—CH₂)₂N—, (CH₂═CH—CH₂)₂N—CO, CH₂═CW¹—CO—NH—,CH₂═CH—(COO)_(k1)-Phe-(O)_(k2)—, CH₂═CH—(CO)_(k1)-Phe(O)_(k2)—,Phe-CH═CH— and W⁴W⁵W⁶Si—, in which W¹ denotes H, F, Cl, CN, CF₃, phenylor alkyl having 1 to 5 C atoms, in particular H, F, Cl or CH₃, W² and W³each, independently of one another, denote H or alkyl having 1 to 5 Catoms, in particular H, methyl, ethyl or n-propyl, W⁴, W⁵ and W⁶ each,independently of one another, denote Cl, oxaalkyl or oxacarbonylalkylhaving 1 to 5 C atoms, W⁷ and W⁸ each, independently of one another,denote H, Cl or alkyl having 1 to 5 C atoms, Phe denotes 1,4-phenylene,k₁, k₂ and k₃ each, independently of one another, denote 0 or 1, k₃preferably denotes 1, and k₄ denotes an integer from 1 to 10.

Very particularly preferred groups P are selected from the groupconsisting of CH₂═CW¹—CO—O—, in particular CH₂═CH—CO—O—,CH₂═C(CH₃)—CO—O— and CH₂═CF—CO—O—, furthermore CH₂═CH—O—,(CH₂═CH)₂CH—O—CO—, (CH₂═CH)₂CH—O—,

Further preferred polymerizable groups P are selected from the groupconsisting of vinyloxy, acrylate, methacrylate, fluoroacrylate,chloroacrylate, oxetane and epoxide, most preferably from acrylate andmethacrylate.

If Sp is different from a single bond, it is preferably of the formulaSp″-X″, so that the respective radical P-Sp- conforms to the formulaP-Sp″-X″—, wherein

-   Sp″ denotes alkylene having 1 to 20, preferably 1 to 12, C atoms,    which is optionally mono- or polysubstituted by F, Cl, Br, I or CN    and in which, in addition, one or more non-adjacent CH₂ groups may    each be replaced, independently of one another, by —O—, —S—, —NH—,    —N(RO)—, —Si(R⁰R⁰⁰)—, —CO—, —CO—O—, —O—CO—, —O—CO—O—, —S—CO—,    —CO—S—, —N(R⁰⁰)—CO—O—, —O—CO—N(R⁰)—, —N(R⁰)—CO—N(R⁰⁰)—, —CH═CH— or    —C≡C— in such a way that 0 and/or S atoms are not linked directly to    one another,-   X″ denotes —O—, —S—, —CO—, —CO—O—, —O—CO—, —O—CO—O—, —CO—N(R⁰)—,    —N(R⁰)—CO—, —N(R⁰)—CO—N(R⁰⁰)—, —OCH₂—, —CH₂O—, —SCH₂—, —CH₂S,    —CF₂O—, —OCF₂—, —CF₂S—, —SCF₂—, —CF₂CH₂—, —CH₂CF₂—, —CF₂CF₂—,    —CH═N—, —N═CH—, —N═N—, —CH═CR⁰—, —CY²═CY³—, —C≡C—, —CH═CH—COO—,    —O—CO—CH═CH— or a single bond,-   R⁰ and R⁰⁰ each, independently of one another, denote H or alkyl    having 1 to 20 C atoms, and-   Y² and Y³ each, independently of one another, denote H, F, Cl or CN.-   X″ is preferably —O—, —S—, —CO—, —COO—, —OCO—, —O—COO—, —CO—NR⁰—,    —NR⁰—CO—, —NR⁰—CO—NR⁰⁰— or a single bond.

Typical spacer groups Sp and -Sp″-X″— are, for example, —(CH₂)_(p1)—,—(CH₂CH₂O)_(p1)—CH₂CH₂—, —CH₂CH₂—S—CH₂CH₂—, —CH₂CH₂—NH—CH₂CH₂— or—(SiR⁰R⁰⁰—O)_(p1)—, in which p1 is an integer from 1 to 12, q1 is aninteger from 1 to 3, and R⁰ and R⁰⁰ have the meanings indicated above.

Particularly preferred groups Sp and -Sp″-X″— are —(CH₂)_(p1)—,—(CH₂)_(p1)—O—, —(CH₂)_(p1)—O—CO—, —(CH₂)_(p1)—CO—O—,—(CH₂)_(p1)—O—CO—O—, in which p1 and q1 have the meanings indicatedabove.

Particularly preferred groups Sp″ are, in each case straight-chain,ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene,nonylene, decylene, undecylene, dodecylene, octadecylene,ethyleneoxyethylene, methyleneoxybutylene, ethylenethioethylene,ethylene-N-methyliminoethylene, 1-methylalkylene, ethenylene,propenylene and butenylene.

Very preferred compounds of formula M are selected from the followingformulae:

in which the individual radicals, on each occurrence identically ordifferently, and each, independently of one another, have the followingmeaning:

-   P¹, P², P³ a polymerisable group, preferably selected from vinyloxy,    acrylate, methacrylate, fluoroacrylate, chloroacrylate, oxetane and    epoxy,-   Sp¹, Sp², Sp³ a single bond or a spacer group where, in addition,    one or more of the radicals P¹-Sp¹-, P¹-Sp²- and P³-Sp³- may denote    R^(aa), with the proviso that at least one of the radicals P¹-Sp¹-,    P²-Sp² and P³-Sp³- present is different from R^(aa), preferably    —(CH₂)_(p1)—, —(CH₂)_(p1)—O—, —(CH₂)_(p1)—CO—O— or    —(CH₂)_(p1)—O—CO—O— bedeuten, wherein p1 is an integer from 1 to 12,-   R^(aa) H, F, Cl, CN or straight-chain or branched alkyl having 1 to    25 C atoms, in which, in addition, one or more non-adjacent CH₂    groups may each be replaced, independently of one another, by    —C(R⁰)═C(R⁰⁰)—, —C≡C—, —N(R⁰)—, —O—, —S—, —CO—, —CO—O—, —O—CO—,    —O—CO—O— in such a way that 0 and/or S atoms are not linked directly    to one another, and in which, in addition, one or more H atoms may    be replaced by F, Cl, CN or P¹-Sp¹-, particularly preferably    straight-chain or branched, optionally mono- or polyfluorinated    alkyl, alkoxy, alkenyl, alkynyl, alkylcarbonyl, alkoxycarbonyl,    alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 12 C atoms (where    the alkenyl and alkynyl radicals have at least two C atoms and the    branched radicals have at least three C atoms), and wherein R^(aa)    does not denote or contain a group P¹, P² or P³,-   R⁰, R⁰⁰ H or alkyl having 1 to 12 C atoms,-   R^(y) and R^(z) H, F, CH₃ or CF₃,-   X¹, X², X³—CO—O—, —O—CO— or a single bond,-   Z^(M1)—O—, —CO—, —C(R^(y)R^(z))— or —CF₂CF₂—,-   Z^(M2), Z^(M3)—CO—O—, —O—CO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂— or    —(CH₂)_(n)—, where n is 2, 3 or 4,-   L F, Cl, CN or straight-chain or branched, optionally mono- or    polyfluorinated alkyl, alkoxy, alkenyl, alkynyl, alkylcarbonyl,    alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 12    C atoms,-   L′, L″ H, F or Cl,-   k 0 or 1,-   r 0, 1, 2, 3 or 4,-   s 0, 1, 2 or 3,-   t 0, 1 or 2,-   x 0 or 1.

Very preferred are compounds of formulae M2 and M13, especiallydireactive compounds containing exactly two polymerizable groups P¹ andP².

Further preferred are compounds selected from formulae M17 to M32, inparticular from formulae M20, M22, M24, M27, M30 and M32, especiallytrireactive compounds containing exactly three polymerizable groups P¹,P² and P³.

In the compounds G of formulae M1 to M31 the group

is preferably

wherein L on each occurrence, identically or differently, has one of themeanings given above or below, and is preferably F, Cl, CN, NO₂, CH₃,C₂H₅, C(CH₃)₃, CH(CH₃)₂, CH₂CH(CH₃)C₂H₅, OCH₃, OC₂H₅, COCH₃, COC₂H₅,COOCH₃, COOC₂H₅, CF₃, OCF₃, OCHF₂, OC₂F₅ or P-Sp-, very preferably F,Cl, CN, CH₃, C₂H₅, OCH₃, COCH₃, OCF₃ or P-Sp-, more preferably F, Cl,CH₃, OCH₃, COCH₃ or OCF₃, most preferably F or OCH₃.

Preferred compounds of formulae M1 to M32 are those wherein P¹, P² andP³ denote an acrylate, methacrylate, oxetane or epoxy group, verypreferably an acrylate or methacrylate group, most preferably amethacrylate group.

Further preferred compounds of formulae M1 to M32 are those wherein Sp¹,Sp² and Sp³ are a single bond.

Further preferred compounds of formulae M1 to M32 are those wherein oneof Sp¹, Sp² and Sp^(a) is a single bond and another one of Sp¹, Sp² andSp^(a) is different from a single bond.

Further preferred compounds of formulae M1 to M32 are those whereinthose groups Sp¹, Sp² and Sp^(a) that are different from a single bonddenote —(CH₂)_(s1)—X″—, wherein s1 is an integer from 1 to 6, preferably2, 3, 4 or 5, and X″ is X″ is the linkage to the benzene ring and is—O—, —O—CO—, —CO—O, —O—CO—O— or a single bond.

Further preferred compounds of formula M are those selected from Table Ebelow, especially those selected from the group consisting of formulaeRM1, RM-4, RM-8, RM-17, RM-19, RM-35, RM-37, RM-39, RM-40, RM-41, RM48,RM-52, RM-54, RM-57, RM-64, RM-74, RM-76, RM-88, RM-92, RM102, RM-103,RM-109, RM-116, RM-117, RM-120, RM-121, RM-122, RM139, RM-142, RM-153,RM-155, RM-158 and RM-159.

Particularly preferred are LC media comprising one, two or threepolymerizable compounds of formula M.

Further preferred are LC media comprising two or more direactivepolymerizable compounds of formula M, preferably selected from formulaeM1 to M16, very preferably selected from formulae M2 and M13.

Further preferred are LC media comprising one or more direactivepolymerizable compounds of formula M, preferably selected from formulaeM1 to M16, very preferably from formulae M2 and M13, and one or moretrireactive polymerizable compounds of formula M, preferably selectedfrom formulae M17 to M32, very preferably from formulae M20, M22, M24,M27, M30 and M32.

Further preferred are LC media comprising one or more polymerizablecompounds of formula M wherein at least one r is not 0, or at least oneof s and t is not 0, very preferably selected from formulae M2, M13,M22, M24, M27, M30 and M32, and wherein L is selected from the preferredgroups shown above, most preferably from F and OCH₃.

Further preferred are polymerizable compounds, preferably selected fromformula M, very preferably from formulae M1 to M32, most preferably fromthe group consisting of the above-mentioned formulae from Table E, whichshow absorption in the wavelength range from 320 to 380 nm.

Preferably the proportion of the polymerizable compounds in the LCmedium is from 1 to <3%, more preferably from 2 to <3%, very preferablyfrom 2 to 2.5%, most preferably from 2 to 2.2%.

In another preferred embodiment the LC medium contains one or morepolymerization initiators.

Suitable conditions for polymerization and suitable types and amounts ofinitiators are known to the person skilled in the art and are describedin the literature. Suitable for free-radical polymerization are, forexample, the commercially available photoinitiators Irgacure651@,Irgacure184@, Irgacure907®, Irgacure369@ or Darocure1173@ (Ciba AG).

If a polymerization initiator is added to the LC medium, its proportionis preferably from 0.001 to 1% by weight, particularly preferably from0.001 to 0.5% by weight.

Furthermore, it is possible to add to the LC media, for example, 0 to15% by weight of pleochroic dyes, furthermore nanoparticles, conductivesalts, preferably ethyldimethyldodecylammonium 4-hexoxybenzoate,tetrabutylammonium tetraphenylborate or complex salts of crown ethers(cf., for example, Haller et al., Mol. Cryst. Liq. Cryst. 24, 249-258(1973)), for improving the conductivity, or substances for modifying thedielectric anisotropy, the viscosity and/or the alignment of the nematicphases. Substances of this type are described, for example, in DE-A 2209 127, 22 40 864, 23 21 632, 23 38 281, 24 50 088, 26 37 430 and 28 53728.

In the present application and in the examples below, the structures ofthe LC compounds are indicated by means of acronyms, the transformationinto chemical formulae taking place in accordance with Table A. Allradicals C_(n)H_(2n+1) and C_(m)H_(2m+1) are straight-chain alkylradicals having n and m C atoms respectively; n, m and k are integersand preferably denote 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12. Thecoding in Table B is self-evident. In Table A, only the acronym for theparent structure is indicated. In individual cases, the acronym for theparent structure is followed, separated by a dash, by a code for thesubstituents R¹*, R²*, L¹* and L²*:

Code for R^(1*), R^(2*), L^(1*), L^(2*), L^(3*) R^(1*) R^(2*) L^(1*)L^(2*) nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H nOm C_(n)H_(2n+1)OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H n C_(n)H_(2n+1)CN H H nN.F C_(n)H_(2n+1) CN F H nN.F.F C_(n)H_(2n+1) CN F F nFC_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nOF OC_(n)H_(2n+1) F H HnF.F C_(n)H_(2n+1) F F H nF.F.F C_(n)H_(2n+1) F F F nOCF₃ C_(n)H_(2n+1)OCF₃ H H nOCF₃.F C_(n)H_(2n+1) OCF₃ F H n-Vm C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m+1) H H Code for R^(1*) R^(2*) L^(1*) L^(2*) R^(1*),R^(2*), L^(1*), L^(2*), L^(3*) nV-Vm C_(n)H_(2n+1)—CH═CH——CH═CH—C_(m)H_(2m+1) H H

Preferred mixture components are shown in Tables A and B.

TABLE A

TABLE B In the following formulae, n and m each, independently of oneanother, denote 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12, inparticular 2, 3, 5, further- more 0, 4, 6.

Particular preference is given to LC media which, besides the compoundsof the formula I, comprise at least one, two, three, four or morecompounds from Table B.

TABLE C Table C indicates possible dopants which are generally added tothe LC media according to the invention. The LC media preferablycomprise 0-10% by weight, in particular 0.01-5% by weight andparticularly preferably 0.01- 3% by weight of dopants.

TABLE D Stabilisers, which can additionally be added, for example, tothe LC media according to the invention in amounts of 0-10% by weight,are mentioned below.

TABLE E Table E shows illustrative reactive mesogenic compounds (RMs)which can be used in the LC media in accordance with the presentinvention.

RM-1

RM-2

RM-3

RM-4

RM-5

RM-6

RM-7

RM-8

RM-9

RM-10

RM-11

RM-12

RM-13

RM-14

RM-15

RM-16

RM-17

RM-18

RM-19

RM-20

RM-21

RM-22

RM-23

RM-24

RM-25

RM-26

RM-27

RM-28

RM-29

RM-30

RM-31

RM-32

RM-33

RM-34

RM-35

RM-36

RM-37

RM-38

RM-39

RM-40

RM-41

RM-42

RM-43

RM-44

RM-45

RM-46

RM-47

RM-48

RM-49

RM-50

RM-51

RM-52

RM-53

RM-54

RM-55

RM-56

RM-57

RM-58

RM-59

RM-60

RM-61

RM-62

RM-63

RM-64

RM-65

RM-66

RM-67

RM-68

RM-69

RM-70

RM-71

RM-72

RM-73

RM-74

RM-75

RM-76

RM-77

RM-78

RM-79

RM-80

RM-81

RM-82

RM-83

RM-84

RM-85

RM-86

RM-87

RM-88

RM-89

RM-90

RM-91

RM-92

RM-93

RM-94

RM-95

RM-96

RM-97

RM-98

RM-99

RM-100

RM-101

RM-102

RM-103

RM-104

RM-105

RM-106

RM-107

RM-108

RM-109

RM-110

RM-111

RM-112

RM-113

RM-114

RM-115

RM-116

RM-117

RM-118

RM-119

RM-120

RM-121

RM-122

RM-123

RM-124

RM-125

RM-126

RM-127

RM-128

RM-129

RM-130

RM-131

RM-132

RM-133

RM-134

RM-135

RM-136

RM-137

RM-138

RM-139

RM-140

RM-141

RM-142

RM-143

RM-144

RM-145

RM-146

RM-147

RM-148

RM-149

RM-150

RM-151

RM-152

RM-153

RM-154

RM-155

RM-156

In a preferred embodiment, the mixtures according to the inventioncomprise one or more polymerisable compounds, preferably selected fromthe polymerisable compounds of the formulae RM-1 to RM-144. Of these,compounds RM-1, RM-4, RM-8, RM-17, RM-19, RM-35, RM-37, RM-39, RM-40,RM-41, RM-48, RM-52, RM-54, RM-57, RM-64, RM-74, RM-76, RM88, RM-92,RM-102, RM-103, RM-109, RM-116, RM-117, RM-120, RM-121, RM-122, RM-139,RM-142, RM-153, RM-155, RM-158 and RM-159 are particularly preferred.

The following examples are intended to explain the invention withoutlimiting it. Above and below, unless explicitly noted otherwise, allpercentage data denote percent by weight, and relate to thecorresponding mixture as a whole, comprising all solid orliquid-crystalline components, without solvents. Furthermore, unlessexplicitly noted otherwise, all temperatures are indicated in in degreesCelsius (° C.). m.p. denotes melting point, cl.p.=clearing point.

Furthermore, C=crystalline state, N=nematic phase, S=smectic phase andI=isotropic phase. The data between these symbols represent thetransition temperatures.

In addition, the following abbreviations and symbols are used:

-   V₀ threshold voltage, capacitive [V] at 20° C.,-   n_(e) extraordinary refractive index at 20° C. and 589 nm,-   n_(o) ordinary refractive index at 20° C. and 589 nm,-   Δn optical anisotropy at 20° C. and 589 nm,-   ε_(⊥) dielectric permittivity perpendicular to the director at    20° C. and 1 kHz,-   ε_(∥) dielectric permittivity parallel to the director at 20° C. and    1 kHz,-   Δε dielectric anisotropy at 20° C. and 1 kHz,-   cl.p., T_(ni) clearing point [° C.],-   γ₁ rotational viscosity at 20° C. [mPa·s],-   K₁ elastic constant, “splay” deformation at 20° C. [pN],-   K₂ elastic constant, “twist” deformation at 20° C. [pN],-   K₃ elastic constant, “bend” deformation at 20° C. [pN].

All physical properties are and have been determined in accordance with“Merck Liquid Crystals, Physical Properties of Liquid Crystals”, StatusNovember 1997, Merck KGaA, Germany, and apply for a temperature of 20°C. unless explicitly indicated otherwise in each case.

The term “threshold voltage” for the present invention relates to thecapacitive threshold (V⁰), also known as the Freedericks threshold,unless explicitly indicated otherwise. In the examples, the opticalthreshold may also, as generally usual, be quoted for 10% relativecontrast (Via).

EXAMPLE 1

The LC mixture N1 is formulated as follows:

APUQU-3-F 9.0% cl.p. 92.6° C. CC-3-V 15.0% Δn 0.1454 CC-3-V1 8.0% Δε +11.3 CCH-35 7.5% ε_(||) 14.8 CPGP-5-2 6.0% γ₁ 119 mPa·s CPGP-5-3 5.0% K₁15.3 CPP-3-F 8.0% K₃ 14.9 PCH-301 4.5% V₀ 1.23 V PGP-2-3 8.0% PGUQU-3-F9.5% PP-1-2V1 2.5% PPGU-3-F 0.5% PUQU-3-F 16.5%

To 99.28% of the mixture N1 are added 0.72% of the chiral dopant S-4011.

EXAMPLE 2

The LC mixture N2 is formulated as follows:

APUQU-2-F 6.0% cl.p. 92.8° C. APUQU-3-F 6.0% Δn 0.1119 BCH-32 4.5% Δε11.5 CC-3-V 33.5% ε_(||) 15.0 CC-3-V1 1.0% γ₁ 96 mPa·s CCP-V-1 10.0% K₁13.4 CCP-V2-1 10.0% K₃ 16.5 CDUQU-3-F 6.0% V₀ 1.14V CPGP-5-2 2.5%PGUQU-3-F 4.0% PGUQU-4-F 4.0% PPGU-3-F 0.5% PUQU-3-F 12.0%

To 99.75% of the mixture N2 are added 0.22% of the chiral dopant S-4011and 0.03% of the stabiliser S1-1.

EXAMPLE 3

The LC mixture N3 is formulated as follows:

APUQU-2-F 6.0% cl.p. 92.8° C. APUQU-3-F 6.0% Δn 0.1119 BCH-32 4.5% Δε11.5 CC-3-V 33.5% ε_(||) 15.0 CC-3-V1 1.0% γ₁ 96 mPa·s CCP-V-1 10.0% K₁13.4 CCP-V2-1 10.0% K₃ 16.5 CDUQU-3-F 6.0% V₀ 1.14 V CPGP-5-2 2.5%PGUQU-3-F 4.0% PGUQU-4-F 4.0% PPGU-3-F 0.5% PUQU-3-F 12.0%

To 99.888% of the mixture N3 are added 0.04% of the stabiliser S2-1 and0.108% of the chiral dopant S-4011.

1. An LC medium with positive dielectric anisotropy, characterised inthat it contains one or more compounds of formula I, one or morecompounds selected from formulae II and III, and one or more chiraldopants

wherein the individual radicals, independently of each other and on eachoccurrence identically or differently, have the following meanings“alkyl” C₁₋₆-alkyl, R^(a1) H, CH₃ or C₂H₅, i, k 0, 1, 2 or 3,

R⁰ an unsubstituted or halogenated alkyl or alkoxy radical having 1 to15 C atoms, where, in addition, one or more CH₂ groups in these radicalsmay each be replaced, independently of one another, by —C≡C—, —CF₂O—,—CH═CH—,

—CO—O— or —O—CO— in such a way that O atoms are not linked directly toone another, or denotes,

X⁰ F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, a halogenatedalkenyl radical, a halogenated alkoxy radical or a halogenatedalkenyloxy radical having up to 6 C atoms, and Y⁰ H or CH₃, Y¹⁻⁶ H or F.2. The LC medium according to claim 1, characterized in that it containsone or more chiral dopants selected from the group consisting of R- orS-1011, R- or S-2011, R- or S-3011, R- or S-4011, and R- or S-5011


3. The LC medium according to claim 1, characterized in that itcomprises one or more compounds of the formula I selected from thefollowing subformulae

wherein “alkyl” has the meaning given in claim
 1. 4. The LC mediumaccording to claim 1, characterized in that it comprises one or morecompounds of formula I selected from the group consisting of thefollowing subformulae:


5. The LC medium according to claim 1, characterized in that itcomprises one or more compounds selected from the group consisting ofthe following subformulae

in which R⁰ and X⁰ have the meanings given in claim
 1. 6. The LC mediumaccording to claim 1, characterized in that it comprises one or morecompounds selected from the group consisting of the followingsubformulae


7. The LC medium according to claim 1, a characterized in that itadditionally comprises one or more compounds selected from the groupconsisting of the following formulae

in which R⁰, X⁰ and Y¹⁻⁴ have the meanings given in claim 1, Z⁰ denotes—C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—, —CF₂CH₂—, —CH₂O—,—OCH₂—, —COO— or —OCF₂—, in formulae V and VI also a single bond, informulae V and VIII also —CF₂O—, r denotes 0 or 1, and s denotes 0 or 1.8. The LC medium according to claim 1, characterised in that itcomprises one or more compounds selected from group consisting of thefollowing formulae

in which R⁰ has the meanings given in claim
 1. 9. The LC mediumaccording to claim 1, characterised in that it additionally comprisesone or more compounds selected from the following formulae:

in which X⁰ has the meanings given in claim 1, and “alkyl” denotesC₁₋₆-alkyl, “alkenyl” and “alkenyl*” denote independently of each otherC₂₋₆-alkenyl, L denotes H or F, R″ denotes C₁₋₆-alkyl, C₁₋₆-alkoxy orC₂₋₆-alkenyl.
 10. The LC medium according to claim 1, characterised inthat it comprises one or more compounds selected from the followingsubformulae:

wherein “alkyl” denotes C₁₋₆-alkyl.
 11. The LC medium according to claim1, characterised in that it comprises one or more compounds of thefollowing formula:

in which R¹ and R² independently of each other denote alkyl having 1 to6 C atoms.
 12. The LC medium according to claim 1, characterised in thatit additionally comprises one or more compounds selected from thefollowing formulae:

in which R³ and R⁴ each, independently of one another, denote n-alkyl,alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having up to 6 C atoms,and preferably each, independently of one another, denote alkyl having 1to 6 C atoms or alkenyl having 2 to 6 C atoms.
 13. The LC mediumaccording to claim 1, characterised in that it additionally comprisesone or more compounds selected from the following subformulae:

in which “alkyl” denotes C₁₋₆-alkyl.
 14. The LC medium according toclaim 1, characterised in that it comprises one or more compounds of theformula XVI

in which R³ and R⁴, each independently of one another, denote n-alkyl,alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having up to 6 C atomsand L denotes H or F.
 15. The LC medium according to claim 1,characterised in that it comprises a compound of formula XVIc2


16. The LC medium according to claim 1, characterised in that itcomprises one or more compounds of formula IA1

in which R⁰ is ethyl or propyl and X⁰ is F.
 17. The LC medium accordingto claim 1, characterised in that it comprises one or more compoundsselected from the group consisting of the compounds of the followingformulae

in which R⁰, X⁰ and Y¹⁻⁴ each, independently of one another, have one ofthe meanings given in claim
 1. 18. The LC medium according to claim 1,characterised in that it comprises one or more compounds of the formulaXXIa

in which R⁰ denotes ethyl, n-propyl, n-butyl or n-pentyl.
 19. The LCmedium according to claim 1, characterised in that it comprises one ormore compounds selected from the group of the compounds of the followingformulae

in which R³ denotes n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyl,each having up to 6 C atoms, and X⁰ has the meaning indicated inclaim
 1. 20. The LC medium according to claim 1, characterised in thatit comprises one or more compounds of the formula XXIXa

in which R³ denotes ethyl, n-propyl, n-butyl or n-pentyl.
 21. The LCmedium according to claim 1, characterised in that it comprises one ormore stabilisers.
 22. The LC medium according to claim 1, characterisedin that it comprises one or more stabilisers selected from the groupconsisting of the following formulae

wherein the individual radicals, independently of each other and on eachoccurrence identically or differently, have the following meaningsR^(a-d) straight-chain or branched alkyl with 1 to 10, preferably 1 to6, very preferably 1 to 4 C atoms, most preferably methyl, X^(S) H, CH₃,OH or O*, A^(S) straight-chain, branched or cyclic alkylene with 1 to 20C atoms which is optionally substituted, n an integer from 1 to 6,preferably
 3. 23. The LC medium according to claim 1, characterised inthat it comprises one or more stabilisers selected from the groupconsisting of the following formulae


24. A process for the preparation of an LC medium according to claim 4,characterised in that one or more compounds of the formula I or itssubformulae are mixed with one or more compounds according to claim 4.25. (canceled)
 26. An LC display containing an LC medium according toclaim
 1. 27. The LC display according to claim 26, characterized in thatit is a TN or TN-TFT display.
 28. The LC display according to claim 26,characterized in that it is an LCOS display.
 29. The LC displayaccording to claim 26, characterized in that it comprises asilicon-containing backplane (1), a reflective coating (2) in the shapeof a pixel array, first (3) and second (4) alignment layers providingplanar alignment, a layer (5) of the LC medium, a transparent electrodelayer (6), and a transparent substrate (7).
 30. The LC display accordingto claim 26, characterized in that the helical twisting power and amountof the chiral dopant in the LC medium are selected such that the ratiod/p in the display is from 0.015 to 0.2.